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<p>在<a href="translation_phases.html" title="c/language/translation phases">翻译单元</a>的顶层（及在预处理器后拥有所有 #include 的源文件），每个 C 程序都是<a href="declarations.html" title="c/language/declarations">声明</a>的序列，它们声明函数和拥有<a href="storage_duration.html" title="c/language/storage duration">外部链接</a>的对象。这些声明被称作<i>外部声明</i>，因为它们出现于任何函数的外部。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw2">extern</span> <span class="kw4">int</span> n<span class="sy4">;</span> <span class="co1">// 外部声明拥有外部链接</span>
<span class="kw4">int</span> b <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>    <span class="co1">// 外部定义拥有外部链接</span>
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>c <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// 外部定义拥有内部链接</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>  <span class="co1">// 外部定义拥有外部链接</span>
    <span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 非外部</span>
    <span class="kw1">return</span> b<span class="sy4">;</span> 
<span class="br0">}</span>
<span class="kw4">static</span> <span class="kw4">void</span> x<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 外部定义拥有内部链接</span>
<span class="br0">}</span></pre></div></div>
<p>声明为拥有外部声明的对象拥有静态<a href="storage_duration.html" title="c/language/storage duration">存储期</a>，从而不能使用 <code>auto</code> 或 <code>register</code> 说明符。这些由外部声明引入的标识符拥有<a href="scope.html" title="c/language/scope">文件作用域</a>。
</p>
<h3><span class="mw-headline" id=".E8.AF.95.E6.8E.A2.E6.80.A7.E5.AE.9A.E4.B9.89">试探性定义</span></h3>
<p><i>试探性定义</i>是没有初始化器的外部声明，且要么没有<a href="storage_duration.html" title="c/language/storage duration">存储类说明符</a>或拥有说明符 <code>static</code> 。
</p><p><i>试探性定义</i>是可能或可能不表现为定义的声明。若在同一翻译单元的前方或后方能找到实际的外部定义，则试探性定义仅表现为声明。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> i1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>     <span class="co1">// 定义，外部链接</span>
<span class="kw4">int</span> i1<span class="sy4">;</span>         <span class="co1">// 试探性定义，表现为声明，因为 i1 已定义</span>
<span class="kw2">extern</span> <span class="kw4">int</span> i1<span class="sy4">;</span>  <span class="co1">// 声明，引用前面的定义</span>
 
<span class="kw2">extern</span> <span class="kw4">int</span> i2 <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// 定义，外部链接</span>
<span class="kw4">int</span> i2<span class="sy4">;</span>            <span class="co1">// 试探性定义，表现为声明，因为 i2 已定义</span>
<span class="kw2">extern</span> <span class="kw4">int</span> i2<span class="sy4">;</span>     <span class="co1">// 声明，引用到前面的外部链接定义</span></pre></div></div>
<p>若在同一翻译单元中无定义，则试探性定义表现为拥有初始化器 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy1">=</span> <span class="nu0">0</span></span></span> （对于数组、结构、联合类型则是 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy1">=</span> <span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span></span></span> ）的实际定义。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> i3<span class="sy4">;</span>        <span class="co1">// 试探性定义，外部链接</span>
<span class="kw4">int</span> i3<span class="sy4">;</span>        <span class="co1">// 试探性定义，外部链接</span>
<span class="kw2">extern</span> <span class="kw4">int</span> i3<span class="sy4">;</span> <span class="co1">// 声明，外部链接</span>
<span class="co1">// 在此翻译单元中， i3 被如同“ int i3 = 0; ”的方式定义</span></pre></div></div>
<p>不同于 <a href="storage_duration.html" title="c/language/storage duration">extern</a> 声明，若前一声明已建立标识符链接，则 extern 声明不更改链接；试探性定义可以与同一标识符另一声明的链接不一致。若同一标识符的二个声明均在作用域内且拥有不同链接，则行为未定义：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">static</span> <span class="kw4">int</span> i4 <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// 定义，内部链接</span>
<span class="kw4">int</span> i4<span class="sy4">;</span>            <span class="co1">// 未定义行为：链接与前一行不一致</span>
<span class="kw2">extern</span> <span class="kw4">int</span> i4<span class="sy4">;</span>     <span class="co1">// 声明，引用到内部链接定义</span>
 
<span class="kw4">static</span> <span class="kw4">int</span> i5<span class="sy4">;</span> <span class="co1">// 试探性定义，内部链接</span>
<span class="kw4">int</span> i5<span class="sy4">;</span>        <span class="co1">// 未定义行为：链接与前一行不一致</span>
<span class="kw2">extern</span> <span class="kw4">int</span> i5<span class="sy4">;</span> <span class="co1">// 引用到前者，其链接为内部</span></pre></div></div>
<p>拥有内部链接的试探性定义必须拥有完整类型。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">static</span> <span class="kw4">int</span> i<span class="br0">[</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 错误：试探性 static 声明中的不完整类型</span>
<span class="kw4">int</span> i<span class="br0">[</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// OK，等价于 int i[1] = {0}; 除非在此文件之后重声明</span></pre></div></div>
<h3><span class="mw-headline" id=".E4.B8.80.E4.B8.AA.E5.AE.9A.E4.B9.89.E8.A7.84.E5.88.99">一个定义规则</span></h3>
<p>每个翻译单元可以拥有每个具有<a href="storage_duration.html" title="c/language/storage duration">内部链接</a>（ <code>static</code> 全局名称）的零或一个外部定义。
</p><p>若一个具有内部链接的标识符被用于任何异于<span class="t-rev-inl t-since-c99"><span>非 VLA 的</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span> <a href="sizeof.html" title="c/language/sizeof"><code>sizeof</code></a> <span class="t-rev-inl t-since-c11"><span>，或 <a href="_Alignof.html" title="c/language/ Alignof"><code>_Alignof</code></a> </span> <span><span class="t-mark-rev t-since-c11">(C11 起)</span></span></span>的表达式，则在该翻译单元中必须有且只有一个该标识符的外部定义。
</p><p>整个程序可以拥有每个具有<a href="storage_duration.html" title="c/language/storage duration">外部链接</a>的标识符的零或一个外部定义。
</p><p>若一个具有外部链接的标识符被用于任何异于<span class="t-rev-inl t-since-c99"><span>非 VLA 的</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span> <a href="sizeof.html" title="c/language/sizeof"><code>sizeof</code></a> <span class="t-rev-inl t-since-c11"><span>，或 <a href="_Alignof.html" title="c/language/ Alignof"><code>_Alignof</code></a> </span> <span><span class="t-mark-rev t-since-c11">(C11 起)</span></span></span>的表达式，则在整个程序中必须有且只有一个该标识符的外部定义。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<p>不同翻译单元中的内联定义不受一个定义规则约束。 inline 函数定义的细节见 <a href="inline.html" title="c/language/inline"><code>inline</code></a> 。
</p>
</td>
<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td></tr>
</table>
<p>关键词 <code>extern</code> 与文件作用域中声明在一起的含义，见<a href="storage_duration.html" title="c/language/storage duration">存储期及链接</a>。
</p><p>声明与定义间的区别见<a href="declarations.html#.E5.AE.9A.E4.B9.89" title="c/language/declarations">定义</a>。
</p><p>发明试探性定义是为了标准化各种 C89 前的前置声明具有内部链接标识符的手段。
</p>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 标准（ISO/IEC 9899:2011）：
</li></ul>
<dl><dd><ul><li> 6.9 External definitions (p: 155-159)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 标准（ISO/IEC 9899:1999）：
</li></ul>
<dl><dd><ul><li> 6.9 External definitions (p: 140-144)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 标准（ISO/IEC 9899:1990）： 
</li></ul>
<dl><dd><ul><li> 3.7 EXTERNAL DEFINITIONS 
</li></ul>
</dd></dl>
</div></div>
</div>

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